The present invention relates to a multi-band voltage-controlled oscillator used in a mobile communication apparatus.
As a mobile communication has rapidly spread, a frequency band firstly assigned to it can hardly cover the entire service. Accordingly, an additional band ranging from 1.5 GHz to 2 GHz has now introduced. This requires mobile communication terminals to be compatible with both the frequency bands. Also, as high-frequency semiconductor technologies have significantly advanced, a particular type of a voltage controlled oscillator has been required which can handle at least three different frequency bands for the mobile communication terminals while being formed by a semiconductor integrated circuit.
Such a conventional multi-band voltage-controlled oscillator used in a common mobile communication terminal will be explained. As shown in FIG. 7, the conventional multi-band voltage-controlled oscillator includes a resonant circuit 1 which is switchable between a low frequency band of about 900 MHz and a high frequency band of about 1800 MHz, an oscillator circuit 2 coupled to the resonant circuit 1, a buffer 3 coupled to an output of the oscillator circuit 2, and an output port 4 coupled to an output of the buffer 3. The resonant circuit 1 incorporates a parallel combination of a parallel assembly 7 having a variable-capacitance diode 5 and a capacitor 6 connected in parallel with each other and a serial assembly 10 having inductors 8 and 9 connected in series with each other.
Depending on a controlling voltage supplied from a control port 12, the variable-capacitance diode 5 has the capacitance varied, thus modifying an oscillation frequency continuously. The control port 12 is coupled via a low-pass filter to an output of a PLL circuit.
The switching between two different frequency bands is conducted by a source voltage Vcc or a grounding potential applied to a band switching port 13 to turn on and off an electronic switch 14 connected in parallel with the inductor 9, i.e. both ends of the inductor 9 are short-circuited and disconnected.
More specifically, when a frequency in the high frequency band is oscillated, the electronic switch 14 is turned on to short-circuit between the two ends of the inductor 9. This enables the parallel assembly 7 and the inductor 8 to be connected in parallel with each other and thus to release a frequency of substantially 1800 MHz. Alternatively, when a frequency in the low frequency band is oscillated, the two ends of the inductor 9 are disconnected. This enables the parallel assembly 7 and the serial assembly 10 to be connected in parallel with each other thus releasing a frequency of substantially 900 MHz. Moreover, the oscillator is connected to the PLL circuit as a loop in the mobile communication terminal.
The conventional oscillator allows the electronic switch 14 to be activated with the source voltage Vcc or the grounding potential applied to the band switching port 13. However, particularly when the grounding potential is applied, the potential can hardly present a zero potential to the port, but a small amount of positive potential remaining at the port. This prevents the electronic switch 14 from being completely turned off, thus generating an unstable state. As a result, the oscillation frequency or the thermal characteristics of the oscillator may be inconsistent.
It is an object of the present invention, which solves the problem, to provide a multi-band voltage-controlled oscillator having switching means to be turned on and off securely.
In order to achieve the object of the present invention, a multi-band voltage-controlled oscillator includes a negative source generator coupled to an output of a buffer transistor which delivers an oscillation frequency from an oscillating transistor, second switching means for selectively switching between an output of the negative source generator and a positive source, and mode switching means for receiving an output frequency switching signal received from the outside. At least the oscillating transistor, the buffer transistor, the negative source generator, and the mode switching circuit are integrated into a signal package. Controlling opening and short-circuiting operations of the first switching means with an output of the second switching means provides oscillation outputs in the low frequency band and high frequency band to be released from the output port.
This allows the switching means to be turned on and off securely.